Combined xylene isomerization and transalkylation process unit

ABSTRACT

The xylene isomerization process unit and the transalkylation process units are combined in the present invention. A fractionation column can be shared by the two units, reducing the capital cost of the complex. In some embodiments, a split shell fractionation column and a split separator can be used.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No.61/578,609 which was filed on Dec. 21, 2011.

BACKGROUND OF THE INVENTION

A typical aromatics complex includes a xylene isomerization unit and atransalkylation unit. The xylene isomerization unit and thetransalkylation unit are completely separate process units, each withits own separator, recycle gas compressor, and fractionation column.

An example of a current aromatics complex is shown in FIG. 1. Thecomplex includes a xylene isomerization unit 5 and the transalkylationunit 10. The isomerization unit 5 has an isomerization reactor 15. Thefeed 20 is pre-heated in a heat exchanger 25 and further heated in acharge heater 30 before entering the isomerization reactor 15. Theeffluent 35 from the isomerization reactor 15 is sent to the heatexchanger 25, the condenser 37, and then to the separator 40. Theoverhead 45 from the separator 40 is sent to a recycle gas compressor 50and is recycled back to the isomerization reactor 15. Make-up hydrogen55 is added to the compressed overhead 45 as needed. The liquid 60 fromthe separator 40 is sent to a detoluene fractionation column 65. Theoverhead 70 is sent to the condenser 73, and the overhead receiver 75.The liquid 80 from the overhead receiver 75 is divided into a refluxstream 85 that is sent back to the detoluene fractionation column 65 anda net overhead stream 90 that is sent to a stripper column 95. Theoverhead 100 from the stripper column 95 is combined with the vapor 70from the column 65 and recycled back to the condenser 73 and separator75. Overhead vapor 105 is removed from overhead receiver 75 to maintaincolumn pressure control. The bottoms 110 from the stripper column 95 issent to a benzene column or to an aromatic extraction unit (not shown).The bottoms 115 from the detoluene fractionation column 65 is dividedinto stream 120, which is sent to the reboiler 125 and back to thedetoluene fractionation column 65, and stream 130, which is sent to axylene column (not shown).

The transalkylation unit 10 has a transalkylation reactor 135. The feed140 is pre-heated in a heat exchanger 145 and further heated in a chargeheater 150 before entering the transalkylation reactor 135. The effluent155 from the transalkylation reactor 135 is sent to the heat exchanger145, the condenser 157, and then to the separator 160. The overhead 165from the separator 160 is sent to a recycle gas compressor 170 and isrecycled back to the transalkylation reactor 135. Make-up hydrogen 175is added to the compressed overhead 165 as needed. The liquid 180 fromthe separator 160 is sent to a detoluene fractionation column 185. Theoverhead 190 is sent to a condenser 193, and overhead receiver 195. Theeffluent 200 from the overhead receiver 195 is divided into a refluxstream 205 that is sent back to the detoluene fractionation column 185and a net overhead stream 210 that is sent to a stripper column 215. Theoverhead 220 from the stripper column 215 is combined with the vapor 190from the column 185 and recycled back to the condenser 193 and overheadreceiver 195. Overhead vapor 220 is removed from overhead receiver 195to maintain column pressure control. The bottoms 230 from the strippercolumn 215 is sent to a benzene column or to an aromatic extraction unit(not shown). The bottoms 235 from the detoluene fractionation column 185is divided into stream 240, which is sent to the reboiler 245 and backto the detoluene fractionation column 185, and stream 250, which is sentto a xylene column (not shown).

The duplication of equipment, such as the separators, gas recyclecompressors, and fractionation columns, to process streams containingsimilar components, although in different amounts, adds significantcapital cost to the aromatics complex.

SUMMARY OF THE INVENTION

One aspect of the invention is a combined xylene isomerization andtransalkylation process. In one embodiment, the process includesisomerizing a feed stream in an isomerization reactor in the presence ofan isomerization catalyst under isomerization conditions to produce anisomerization product; transalkylating a feed stream in atransalkylation reactor in the presence of a transalkylating catalystunder transalkylating conditions to produce a transalkylation product;separating the isomerization product in an isomerization separator toproduce an isomerization separator bottoms stream; separating thetransalkylation product in a transalkylation separator to produce atransalkylation separator bottoms stream; providing a split shellfractionation column having a baffle separating the fractionation columninto two sides, the fractionation column having an isomerization inleton the isomerization side of the baffle and a transalkylation inlet onthe transalkylation side of the baffle, the baffle extending from abottom of the column to a location above a highest inlet; introducingthe isomerization separator bottoms stream into the isomerization sideof the fractionation column through the isomerization inlet; introducingthe transalkylation separator bottoms stream into the transalkylationside of the fractionation column through the transalkylation inlet;fractionating the isomerization separator bottom stream in theisomerization side of the fractionation column to produce afractionation column isomerization bottoms stream; and fractionating thetransalkylation separator bottom stream in the transalkylation side ofthe fractionation column to produce a fractionation columntransalkylation bottoms stream.

Another aspect of the invention is a combined xylene isomerization andtransalkylation process unit. In one embodiment, the process unitincludes an isomerization reactor; a transalkylation reactor; anisomerization separator in fluid communication with the isomerizationreactor; a transalkylation separator in fluid communication with thetransalkylation reactor; and a split shell fractionation column having abaffle separating the fractionation column into two sides, thefractionation column having an isomerization inlet on the isomerizationside of the baffle and a transalkylation inlet on the transalkylationside of the baffle, the baffle extending from a bottom of the column toa location above a highest inlet; the isomerization inlet in fluidcommunication with a bottoms stream from the isomerization separator,and the transalkylation inlet in fluid communication with a bottomsstream from the transalkylation separator.

Another aspect of the invention is a split shell fractionation column.In one embodiment, the fractionation unit includes a split shellfractionation column having a baffle separating the column into twosides, the column having a first inlet on the first side of the baffleand a first bottoms outlet on the first side of the column, and a secondinlet on the second side of the baffle, and a second bottoms outlet onthe second side of the column, the baffle extending from a bottom of thecolumn to a location above a highest inlet.

Another aspect of the invention is a split shell separator. In oneembodiment, the split shell separator includes a single separator havinga baffle extending from a bottom of the separator to a location above aliquid level in the separator, a first inlet on a first side of thebaffle, and a first bottoms outlet on the first side of the baffle, anda second inlet on a second side of the baffle, and a second bottomsoutlet on the second side of the baffle.

Another aspect of the invention is a combined xylene isomerization andtransalkylation process. In one embodiment, the process includesisomerizing a feed stream in an isomerization reactor underisomerization conditions to produce an isomerization product;transalkylating a feed stream in a transalkylation reactor undertransalkylating conditions to produce a transalkylation product;combining the isomerization product and the transalkylation product;introducing the combined product into a single separator; separating thecombined product in the separator to produce a separator bottoms stream;and fractionating the separator bottoms stream in a fractionation columnto produce a fractionation column bottoms stream.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is an illustration of a prior art complex having separateisomerization and transalkylation process units.

FIG. 2 is one embodiment of a combined isomerization and transalkylationprocess unit.

FIG. 3 is another embodiment of a combined isomerization andtransalkylation process unit.

FIG. 4 is another embodiment of a combined isomerization andtransalkylation process unit.

DETAILED DESCRIPTION OF THE INVENTION

The xylene isomerization process unit and the transalkylation processunits are combined in the present invention. A single detoluenefractionation column can be shared by the two units. In someembodiments, the separator and recycle gas compressor can also beshared. The removal of one the fractionation columns, as well as one ofthe separators and one of the gas recycle compressors in someembodiments, significantly reduces the capital cost of the unit.

The development of new transalkylation catalysts, which allow thetransalkylation reactor to operate at lower pressures, makes the sharingof equipment between the isomerization section and the transalkylationsection practical.

In one embodiment shown in FIG. 2, the isomerization and transalkylationreactor zones are independent, i.e., each has its own reactor feed heatexchanger, charge heater, reactor, and product condenser. The effluentfrom each reactor section is sent to a common separator.

The isomerization unit 305 has an isomerization reactor 315. The feed320 is pre-heated in a heat exchanger 325 and further heated in a chargeheater 330 before entering the isomerization reactor 315. The effluent335 from the isomerization reactor 315 is sent to the heat exchanger325, and the condenser 337.

The transalkylation unit 310 has a transalkylation reactor 435. The feed440 is pre-heated in a heat exchanger 445 and further heated in a chargeheater 450 before entering the transalkylation reactor 435. The effluent455 from the transalkylation reactor 435 is sent to the heat exchanger445, and the condenser 457.

The effluent 335 from the isomerization reactor 315 is combined with theeffluent 455 from the transalkylation reactor 435 and sent to a commonseparator 340. The overhead 345 from the common separator 340 is sent toa recycle gas compressor 350. A portion 351 of the overhead is recycledback to the isomerization reactor 315, and a portion 353 is recycledback to the transalkylation reactor 435. The recycle can be split usingflow controls valves. Make-up hydrogen 355 is added to the compressedoverhead 345 as needed.

The liquid 360 from the separator 340 is sent to a common detoluenefractionation column 365. The overhead 370 is sent to the condenser 373,and the overhead receiver 375. The liquid 380 from the separator 375 isdivided into a reflux stream 385 that is sent back to the detoluenefractionation column 365 and a net overhead stream 390 that is sent to astripper column 395. The overhead 400 from the stripper column 495 iscombined with the vapor 370 from the column 365 and recycled back to thecondenser 373 and overhead receiver 375. Overhead vapor 405 is removedfrom overhead receiver 375 to maintain column pressure control. Thebottoms 410 from the stripper column 395 is sent to a benzene column orto an aromatic extraction unit (not shown). The bottoms 415 from thedetoluene fractionation column 365 is divided into stream 420, which issent to the reboiler 425 and back to the detoluene fractionation column365, and stream 430, which is sent to a xylene column (not shown).

This approach involves the maximum capital savings because it eliminatesall of the potentially redundant equipment, i.e., separator, recycle gascompressor, and detoluene fractionation column and related equipment.

But the capital saving comes with a substantial energy penalty. The useof the common separator and fractionation column requires mixing thexylene stream from the isomerization reactor with the xylene stream fromthe transalkylation reactor into a single feed stream to the xylenecolumn. However, those streams have significantly different amounts ofA9+ components, with the isomerization stream having very little A9+,and the transalkylation stream having a substantial amount of A9+.Current systems keep the two streams separate and feed them to differenttray locations in the downstream xylene column to reduce the refluxrequired to produce the feed for the xylene separation process and fuelfiring by about 20 to 30%.

The energy problem can be solved by maintaining the separate liquidstreams so that the isomerization stream with low levels of A9+components, and the transalkylation stream with higher levels of A9+components can be fed to the appropriate trays in the xylene column. Inorder to do that, a split-shell fractionation column has been developedwhich segregates the column bottoms product streams.

The split shell fractionation column includes a baffle which divides thetray section into two independent sides. The baffle extends to thebottom of the column, keeping the sump liquid separated. The baffle issolid and comprised of the same material as the fractionation columnshell. The top of the baffle extends to a location above the highestfeed tray. The highest feed tray is the highest feed tray for theisomerization side, the transalkylation side, or any other feed traysthat might be present. It should extend to the level of about half ofthe trays above the highest feed inlet. This would generally be at leastfour trays above the highest feed tray, typically about five trays aboveit. Above means closer to the vapor outlet of the column (or otherequipment), and below means closer to the liquid outlet.

The liquid streams from the isomerization and transalkylation separatorsare fed to opposite sides of the baffle. The baffle does not have todivide the column equally. One side can be larger than the otherdepending on the design of the particular complex and the size of theliquid streams from the isomerization reactor and the transalkylationreactor.

FIG. 3 illustrates one embodiment of a combined isomerization andtransalkylation unit with a split shell fractionation column.

The isomerization unit 505 has an isomerization reactor 515. The feed520 is pre-heated in a heat exchanger 525 and further heated in a chargeheater 550 before entering the isomerization reactor 515. The effluent535 from the isomerization reactor 515 is sent to the heat exchanger525, the condenser 537, and the separator 540.

The overhead 545 from the separator 540 is sent to a recycle gascompressor 550. A portion 551 is recycled back to the isomerizationreactor 515, and a portion 553 is recycled back to the transalkylationreactor 635. The recycle can be split using flow controls valves.Make-up hydrogen 555 can be added to the compressed overhead 545 ifneeded.

The transalkylation unit 510 has a transalkylation reactor 635. The feed640 is pre-heated in a heat exchanger 645 and further heated in a chargeheater 650 before entering the transalkylation reactor 635. The effluent655 from the reactor 635 is sent to the heat exchanger 645, thecondenser 657, and the separator 660.

The overhead 665 from the separator 640 is sent to recycle gascompressor 550.

The fractionation column 565 is a split shell fractionation column. Itincludes a baffle 567 extending from the bottom of the column dividingthe tray section of the fractionation column into two sides. The liquid560 from the separator 540 is sent to one side, and the liquid 680 fromthe separator 660 is sent to the other side. The baffle extends abovethe level of both feed inlets.

The overhead 570 is sent to the condenser 573, and the overhead receiver575. The liquid 580 from the overhead receiver 575 is divided into areflux stream 585 that is sent back to the split shell fractionationcolumn 565 and a net overhead stream 590 that is sent to a strippercolumn 595. The overhead 600 from the stripper column 595 is combinedwith the vapor 570 from the overhead receiver 575 and recycled back tothe condenser 573 and overhead receiver 575. Overhead vapor 600 isremoved from overhead receiver 575 to maintain column pressure control.The bottoms 610 from the stripper column 595 is sent to a benzene columnor to an aromatic extraction unit (not shown).

Because the baffle 567 extends to the bottom of the split shellfractionation column 565, the liquid bottoms from the isomerization sideand the transalkylation side remain separated. The bottoms 615A from theisomerization side of the split shell fractionation column 565 isdivided into stream 620A, which is sent to the reboiler 625A and back tothe isomerization side of the split shell fractionation column 565, andstream 630A, which is sent to a xylene column 685. Stream 630A is theupper feed for the xylene column 685 because of its low A9+ content.

The bottoms 615B from the transalkylation side of the split shellfractionation column 565 is divided into stream 620B, which is sent tothe reboiler 625B and back to the transalkylation side of the splitshell fractionation column 565, and stream 630B, which is sent to thexylene column 685. Stream 630B is the lower feed for the xylene column685 because of its higher A9+ content.

In another embodiment, there is a split separator in addition to thesplit shell fractionation column. This arrangement removes one of theseparators. The split separator has a baffle extending from the bottomto a point near the top of the vessel and substantially above the normallevel of the liquid. The separator does not have to be divided equally;one side can be larger than the other depending on the design of thecomplex and the size of the isomerization and transalkylation streams.

FIG. 4 illustrates one embodiment of a combined isomerization andtransalkylation unit with a split separator and a split shellfractionation column.

The isomerization unit 705 has an isomerization reactor 715. The feed720 is pre-heated in a heat exchanger 725 and further heated in a chargeheater 730 before entering the isomerization reactor 715. The effluent735 from the isomerization reactor 715 is sent to the heat exchanger725, and the condenser 737.

The transalkylation unit 710 has a transalkylation reactor 835. The feed840 is pre-heated in a heat exchanger 845 and further heated in a chargeheater 850 before entering the transalkylation reactor 835. The effluent855 from the transalkylation reactor 835 is sent to the heat exchanger845, and the condenser 857.

The separator 740 has a baffle 741 which divides the separator into twosides. The effluent 735 from the isomerization reactor 715 is sent toone side of the separator 740, while the effluent 855 from thetransalkylation reactor 835 is sent to the other side of the separator740. The baffle 741 extends above the liquid level in the separator andkeeps the liquid from the two sides separated.

The overhead 745 from the separator 740 is sent to a recycle gascompressor 750. A portion 751 is recycled back to the isomerizationreactor 715, and a portion 753 is recycled back to the transalkylationreactor 835. The recycle can be split using flow control valves. Make-uphydrogen 755 is added to the compressed overhead 745 as needed.

The fractionation column 765 is a split shell fractionation columndivided by baffle 767. The liquid 760A from the isomerization side ofseparator 740 is sent to one side of the column, and the liquid 760Bfrom the transalkylation side of separator 740 is sent to the other sideof the column. The baffle extends above the level of both liquid feedinlets.

The overhead 770 is sent to the condenser 773, and the overhead receiver775. The liquid 780 from the overhead receiver 775 is divided into areflux stream 785 that is sent back to the split shell fractionationcolumn 765 and a net overhead stream 790 that is sent to a strippercolumn 795. The overhead 800 from the stripper column 795 is combinedwith the vapor 770 from the column 765 and recycled back to thecondenser 773 and overhead receiver 775. Overhead vapor 805 is removedfrom overhead receiver 775 to maintain column pressure control. Thebottoms 810 from the stripper column 795 is sent to a benzene column orto an aromatic extraction unit (not shown).

Because the baffle 767 extends to the bottom of the split shellfractionation column 765, the liquid bottoms from the isomerization sideand the transalkylation side remain separated. The bottoms 815A from theisomerization side of the split shell fractionation column 765 isdivided into stream 820A, which is sent to the reboiler 825A and back tothe isomerization side of the split shell fractionation column 765, andstream 830A, which is sent to a xylene column 885. Stream 830A is theupper feed for the xylene column 885 because of its low A9+ content.

The bottoms 815B from the transalkylation side of the split shellfractionation column 765 is divided into stream 820B, which is sent tothe reboiler 825B and back to the transalkylation side of the splitshell fractionation column 765, and stream 830B, which is sent to thexylene column 885. Stream 830B is the lower feed for the xylene column885 because of its higher A9+ content.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A combined xylene isomerization andtransalkylation process comprising: isomerizing a feed stream in anisomerization reactor in the presence of an isomerization catalyst underisomerization conditions to produce an isomerization product;transalkylating a feed stream in a transalkylation reactor in thepresence of a transalkylating catalyst under transalkylating conditionsto produce a transalkylation product; separating the isomerizationproduct in an isomerization separator to produce an isomerizationseparator bottoms stream; separating the transalkylation product in atransalkylation separator to produce a transalkylation separator bottomsstream; providing a split shell fractionation column having a baffleseparating the fractionation column into two sides, the fractionationcolumn having an isomerization inlet on the isomerization side of thebaffle and a transalkylation inlet on the transalkylation side of thebaffle, the baffle extending from a bottom of the column to a locationabove a highest inlet; introducing the isomerization separator bottomsstream into the isomerization side of the fractionation column throughthe isomerization inlet; introducing the transalkylation separatorbottoms stream into the transalkylation side of the fractionation columnthrough the transalkylation inlet; fractionating the isomerizationseparator bottom stream in the isomerization side of the fractionationcolumn to produce a fractionation column isomerization bottoms stream;fractionating the transalkylation separator bottom stream in thetransalkylation side of the fractionation column to produce afractionation column transalkylation bottoms stream.
 2. The process ofclaim 1 further comprising: introducing the fractionation columnisomerization bottoms stream into a xylene fractionation column at afirst position; introducing the fractionation column transalkylationbottoms stream into the xylene fractionation column at a second positionbelow the first position; and fractionating the fractionation columnisomerization bottoms stream and the fractionation columntransalkylation bottoms stream in the xylene fractionation column. 3.The process of claim 1 further comprising recycling an overhead streamfrom the isomerization separator to the isomerization reactor, or thetransalkylation reactor, or both.
 4. The process of claim 1 furthercomprising recycling an overhead stream from the transalkylationseparator to the isomerization reactor, or the transalkylation reactor,or both.
 5. The process of claim 1 wherein the isomerization separatorand the transalkylation separator comprise a single separator having abaffle extending from the bottom of the separator to a location abovethe liquid level in the separator.
 6. The process of claim 5 furthercomprising recycling an overhead stream from the single separator to theisomerization reactor, or the transalkylation reactor, or both.
 7. Theprocess of claim 1 further comprising fractionating an overhead streamfrom the fractionation column.